The present invention relates to programmable gain amplifiers (“PGAs”).
Low noise, low power and high accuracy PGAs can be implemented using a chopped differential amplifier with capacitors as feedback elements. This type of PGA is described in U.S. Pat. No. 7,795,960, which is assigned to Analog Devices, Inc., the assignee of the present invention. Capacitors do not transfer DC signals and, therefore, an input chop circuit converts an input DC signal into a square wave. The square wave is gained up by the amplifier with capacitive feedback. An output chop circuit then demodulates the amplified square wave back to a gained up DC signal. Low noise can be achieved because capacitors are noiseless and hence the differential amplifier is the only noise source in the PGA. Gain is accurately defined by the ratio of capacitors, which is stable and can be well controlled in integrated circuit (“IC”) manufacturing processes. Programmable gain can be achieved by changing the values of the capacitors, e.g., by switching capacitors in or out of the amplifier circuit from a bank of capacitors.
Capacitive PGAs, however, do not naturally define the common mode of a voltage presented at the inputs of the differential amplifier, which should be matched to the common mode of the amplifier itself. In order to set this voltage, additional circuitry becomes necessarily required, which typically involves very large resistors and is thus intrinsically slow. Accordingly, the inventor perceives a need in the art for a PGA architecture that defines a common mode voltage at a differential amplifier with faster response than these prior attempts.